The present invention relates to plate-like barium ferrite particles which are suitable as a precursor of a magnetic material for sintered magnets and magnetic recording media and a process for producing the same. The present invention relates, more particularly, to plate-like barium ferrite particles suitable for use in precursor for sintered magnets and magnetic recording media, which have an average particle diameter of not less than 0.03 .mu.m and less than 0.1 .mu.m, which are represented by the following formula (I): EQU BaO.n[(Fe.sub.1-x-y Co.sub.x Ti.sub.y).sub.2 O.sub.3-x+y ] (I)
wherein x and y are independently from 0.02 to 0.12 and n is from 4 to 7 , the atomic ratio of Co to Fe(III) being from 0.02:1 to 0.13:1 and the molar ratio of Ti to Co being from more than 0 to not more than 0.8, and a process for producing plate-like barium ferrite particles suitable for use in precursor for sintered magnets and sintered magnets and magnetic recording media, which are represented by the following formula (I): EQU BaO.n[(Fe.sub.1-x-y Co.sub.x Ti.sub.y).sub.2 O.sub.3-x+y ] (I)
wherein x and y are independently from 0.02 to 0.12 and n is from 4 to 7, the atomic ratio of Co to Fe(III) being from 0.02:1 to 0.13:1 and the molar ratio of Ti to Co being from more than 0 to not more than 0.8, which the process comprises autoclaving at a temperature of from 120.degree. to 330.degree. C. an aqueous alkaline suspension of iron hydride(III) containing Fe(III), Co(II), Ti(IV) and Ba ions, the atomic ratio of the amount of Co to the amount of Fe(III) in the suspension being from 0.02:1 to 0.13:1 and the molar ratio of Ti to Co therein being from more than 0 to not more than 0.8, thereby regulating the average particle diameter to from not less than 0.03 .mu.m to less than 0.1 .mu.m.
An object of the present invention is to produce plate-like barium ferrite particles as a precursor, having a desired average particle diameter in the range of not less than 0.03 to not more than 0.1 .mu.m from an aqueous solution, by autoclaving an aqueous alkaline suspension of iron hydroxide (III) containing Ba ions, a predetermined amount of Co compound and Ti compound.
Plate-like barium ferrite particles according to the present invention are mainly used as a precursor of magnetic materials for sintered magnets, magnetic materials for magnetic recording media, etc.
Barium ferrite particles are at present widely used as a precursor of magnetic materials for sintered magnets.
A sintered magnet is manufactured by pressurizing, molding and baking a magnetic material such as barium ferrite particles.
Also, recently it is required to enhance increasingly recording density in the field of magnetic recording. Much has been expected from plate-like barium ferrite particles as a precursor of magnetic materials for magnetic recording media and some of them have been put to practical use.
In a field of magnetic tapes and magnetic discs, rapid progress has been made in putting perpendicular magnetic type tapes and discs which are capable of recording with more than about three times as high density as conventional ones to practical use. Plate-like barium ferrite magnetic particles, which have an easy magnetization axis perpendicular to the plate surface, have been expected as a magnetic material for magnetic recording media.
In a field of magnetic cards, plate-like barium ferrite magnetic particles, which have a higher coercive force than acicular magnetic iron oxide particles and Co-deposited or Co-coated acicular magnetic iron oxide particles which have conventionally been used, are widely used as a magnetic material for magnetic recording media.
Since plate-like barium ferrite magnetic particles have been expected to be used in various fields, as described above, the property of the plate-like barium ferrite particles as a precursor thereof which is commonly required in each field is fineness.
As the plate-like barium ferrite magnetic particles are finer, their surface activity thereof becomes larger and they are easier to sinter. Therefore, it is possible to lower the sintering temperature, which is not only advantageous industrially and economically but also produces a product having a uniform composition.
In a field of magnetic recording media such as magnetic tapes, magnetic discs and magnetic cards, it is necessary to make the particle size of a magnetic material for magnetic recording media very small in order to reduce noise of a recording medium and, in particular, plate-like barium ferrite magnetic particles having a particle diameter of about not greater than 0.1 .mu.m have been strongly demanded.
Some barium ferrite magnetic particles and processes for producing barium ferrite magnetic particles have conventionally been proposed.
Some of the typical ones are plate-like barium ferrite magnetic particles which are represented by the formula BaCo.sub.x Ti.sub.y Fe.sub.12-x-y O.sub.19 wherein x and y independently are numbers from 0.1 to 0.5 and which have a BET specific surface area of from 20 to 70 m.sup.2 /g, an average particle diameter of from 0.05 to 0.3 .mu.m, a magnetization of larger than 30 emu/g in a magnetic field of 7.96.times.10.sup.5 A m.sup.-1 (10 KOe) and a coercive force of from 2.39.times.10.sup.4 to 7.96.times.10.sup.4 A m.sup.-1 (300 to 1,000 Oe), and a process for producing plate-like barium magnetic ferrite particles represented by the formula: EQU BaCo.sub.x Ti.sub.y Fe.sub.12-x-y O.sub.19
wherein x and y independently are numbers from 0.1 to 0.5, which process comprises autoclaving at a temperature of from 250.degree. to 320.degree. C. an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions and a molar excess with respect to the total amount of Fe(III), Co(II), Ti(IV) and Ba ions of an alkali metal hydroxide and in which suspension the atomic ratio of the total amount of Co(II) and Ti(IV) to the amount of Fe(III) is from 0.017:1 to 0.09:1 and the atomic ratio of Ba ions to the total sum of Fe(III), Co(II) and Ti(IV) is from 1:7 to 1:9 (European Pat. No. 0123445 A).
Another is a process for production of plate-like barium ferrite magnetic particles represented by the formula: EQU BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19
wherein x and y are respectively 0.3 to 1.2 and M is a divalent metal selected from the group consisting of Co, Ni, Zn and Mn for magnetic recording, free of .alpha.-Fe.sub.2 O.sub.3 exhibiting a magnetization of larger than 35 emu/g in a magnetic field of 10 KOe and a coercive force of 500 to 1,000 Oe, which comprises forming an aqueous highly alkaline suspension of particles represented by the formula: EQU Fe.sub.12-x-y M.sub.x Ti.sub.y O.sub.16
wherein x, y and M are the same as defined above, having a spinel crystalline structure and an average particle diameter of less than 0.1 .mu.m and Ba ions where the atomic ratio of Ba to the total amount of the metal in said particles of spinel crystalline structure is in the range of from 1:8 to 1:10 and autoclaving said suspension at a temperature in the range from 250.degree. C.-330.degree. C. to form said plate-like barium ferrite particles (U.S. Pat. No. 4,561,988).
These processes, however, are aimed at reducing a coercive force by adjusting the valence of Fe(III) ions and additives, for example, Co(II), Ti(IV), etc., as is clear from the embodiments, and therefore it is preferable that the amounts of Co(II) compound and Ti(IV) compound to be added are equimolar. On the other hand, in the present invention, the amounts of Co(II) compound and Ti(IV) compound to be added are different and they are added in order to control the particle size of the produced plate-like barium ferrite particles. Accordingly, the conventional product and processes are completely different from the process of the present invention in technical means, objects and effects.
Furthermore, in the above-described autoclaving processes, the particle diameter of the produced plate-like barium ferrite particles in usually about 0.05 to 0.3 .mu.m, and it is difficult to obtain fine particles having an average particle diameter of less than 0.1 .mu.m. It is still more difficult to obtain plate-like barium ferrite particles having a desired average particle diameter of less than 0.1 .mu.m in accordance with a particular purpose.
Accordingly, an offer of plate-like barium ferrite particles as a precursor thereof, having an average particle diameter of not less than 0.03 and less than 0.1 .mu.m and a process for producing the plate-like barium ferrite particles have been strongly demanded.
As a result of various studies on a process for producing plate-like barium ferrite particles as a precursor thereof, having a desired particle size of an average particle diameter of less than 0.1 .mu.m, the present inventors have found that plate-like barium ferrite particles as a precursor thereof, having an average particle diameter of not less than 0 03 .mu.m and less than 0.1 .mu.m are produced by autoclaving at a temperature of from 120.degree. to 330.degree. C. an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions, the atomic ratio of the amount of Co to the amount of Fe(III) in the suspension being from 0.02:1 to 0.13:1 and the molar ratio of Ti to Co therein being from more than 0 to not more than 0.8, and on the basis of the finding the present invention has been attained.